Tuesday, May 31, 2011

The KPAC buzz: 10.5kHz in L-R

I have recorded and analyzed the buzz from KPAC FM during a 1.7 second silent interval. What I hear as the buzz is a 10.5kHz tone mainly in the L-R subcarrier. I will be sending this information to the station engineer shortly.

I connected the fixed outputs of my Pioneer F-26 tuner to the analog inputs of an Alesis Masterlink set to 88kHz 20 bit sampling (I had intended 24 bit but missed that). I recorded several snippets when it seemed like there would shortly be a brief silence. In one of them, I got really lucky, I got 1.7 seconds of pure silence. Or it would have been silence without the noise and buzz. I made CD24 discs of the 88.2/20 bit recordings and loaded the files onto my Mac. I ran the Wave Editor program and created 44.1kHz Wave and AIFF files for later use. I cropped the best example down to the virtually silent portion, with extent of exactly 1.7 seconds.

Even though I could clearly hear the buzz when playing back the file, it was hard to see anything specific in the Wave Editor Spectrum Analyzer. But it became more clear once I created a mono signal using L-R subtraction function. Then, after trying several other digital processing functions in Wave Editor, I got the idea of using the DSP parametric equalizer. I noticed a little bump around 11kHz, and aimed for that. I found I could get nearly perfect cancellation of the buzz by setting the frequency to 10579 Hz (the nearest alternatives in Wave Editor are 9998 and 11194 Hz, which provide considerably less cancellation), the Q to 10, and the amplitude to -20dB. The peak I am cancelling looks to be a bit lower than the 10579 Hz notch. When I set the Q to 20, it doesn't cancel quite as well. So I'm guessing the actual frequency is 10.5kHz, or quite close to that anyway (+/- 300Hz).